Several proposals published in the technical literature of the iron and steelmaking industry and in patents can be found about the utilization of gases derived from the pyrolysis or from gasification of coal (coal gas), of which an example is coke oven gas, for producing direct reduced iron, also known in the industry as sponge iron (DRI in English).
DRI is a granular solid material produced by the reaction of particulate iron ores, mainly iron oxides, in the form of lumps, pellets of concentrated ore, or mixtures thereof, with a reducing gas mainly composed of hydrogen and carbon monoxide, at a temperature in the range from about 750° C. to about 1100° C.
Typical DRI plants are shown for example in U.S. Pat. Nos. 3,779,741; 3,765,872; 4,150,972; 4,336,063; 4,834,792; and 5,078,787. These systems commonly comprise vertical shaft reactors having a reduction zone in their upper part and a discharge zone in their lower part.
The reducing gas fed to the reactor at high temperatures is typically mainly composed of hydrogen and carbon monoxide and after reacting with the iron oxides produces water and carbon dioxide.
Chemical reduction of iron oxides is carried out by such reducing gas is typically produced by the reformation or the partial combustion of natural gas;however, there is an increasing interest in utilizing other gases derived from gasification (partial combustion) of solid and liquid fossil fuels, such as coal, among which coke oven gas is included.
Coke oven gas is a by-product of coking of coal. Coke is used in the steel industry as a feedstock for blast furnaces where pig iron is produced. Pig iron is liquid metallic iron used as raw material for steelmaking. Coal is pyrolyzed in coke ovens, i.e. is heated without presence of oxygen whereby the volatile matters contained in coal are evaporated and are thus removed from coal, producing coke.
The gas effluent from coke ovens at a temperature of about 1100° C. passes to a gas purification plant wherein some of the volatile substances are recuperated. This gas is then cooled and the cooled gas, saturated with water at a temperature of about 80° C. is separated from the cooling water which entrains tars and other condensed organic compounds.
Coke oven gas typically has the following composition in volume % on a dry basis:
Hydrogen55-62%Methane22-26%Nitrogen 8-10%Carbon monoxide6-8%Carbon dioxide2-3%Hydrocarbons (ethane, propane, etc.)2-3%
Additionally, coke oven gas contains other contaminants and undesired substances, whereby this gas has some special characteristics that condition its utilization, such as:    Tar vapors    Light oils vapors (aromatics) mainly comprising benzene, toluene and xylene, which jointly are known as BTX.    Naphthalene vapors    Ammonia    Hydrogen sulfide H2S    Hydrogen cyanide
The concentration levels of the above-mentioned substances in the available coke oven gas, depends on the cleaning process undergone by the gas effluent from coke ovens.
So that the coke oven gas can be utilized as fuel in the coke ovens or in other processes in a steelmaking plant, the gas must be treated for condensed water and other contaminant substances; remove the tar aerosols for avoiding plugging of pipes and equipment; remove ammonia for avoiding corrosion of pipes and gas-handling equipment; remove naphthalene for avoiding deposits and plugging of pipes due to its condensation; remove light oils if there is interest in recuperating BTX; and remove sulfur compounds, for example hydrogen sulfide and mercaptans, for complying with environmental regulations.
Since coke oven gas is mainly composed of H2 and CH4, it has been proposed to utilize it for chemical reduction of iron ores and obtain metallic iron in solid form for increasing steel production.
However, coke oven gas can not be fed directly to the direct reduction plant if it is not previously treated to clean out those substances which may cause damages to the plant equipment or which are noxious pollutants for the environment.
The cleaning and conditioning process for coke oven gas entails a high cost since it implies installation and operation of several chemical plants for cooling the gas stream effluent from coke ovens and for separation of condensed oils and ammonia as well as removal of sulfur compounds. The normal separation processes for thiophene, mercaptans and other aromatic sulfur compounds requires that these substances be previously transformed into hydrogen sulfide (H2S) which is suitable of being absorbed by chemical solvents. This process however also has a high capital and operating cost which can be avoided or significantly lowered by applying the present invention.
The cost of cleaning coke oven gas, mainly the desulfurization and removal of BTX, can be lowered by the present invention, by synergistically utilizing the coke oven gas in a direct reduction plant, not just as a fuel, but instead make its utilization snore cost effective as a chemical agent.
It has been proposed, for example in U.S. Pat. No. 4,270,739, to have a direct reduction process which utilizes coke oven gas for reduction of iron ores to metallic iron, wherein the coke oven gas is heated in a direct fired heater before being introduced to the upper part of a reduction reactor wherein the sulfur compounds are adsorbed by the iron care particles. The coke oven gas, free of sulfur compounds, is withdrawn from said reactor and is fed to a catalytic reformer wherein the hydrocarbons present in the gas are reformed to H2 and CO and are then fed to a lower zone where the reduction of iron ores to metallic iron is effected at a higher temperature than the temperature in the adsorption zone of the reactor. This process has the disadvantage that the heater used for heating the coke oven gas to a temperature of 700° C. or higher is a direct-fired heater and therefore the sulfur compounds cause corrosion of the heating pipes and the high level of hydrocarbons cause carbon deposits in the heating pipes which entail damages and problems in the operation of the heater. Furthermore this patent proposes that the reactor have two gas distributing plenums: one for distributing the hot coke oven gas in the upper zone of the reactor and another for distributing the hot reducing gas produced in the reformer.
U.S. Pat. No. 4,351,513 discloses a process for utilizing coke oven gas in a direct reduction reactor wherein the iron ore fed to the upper part of the reactor adsorbs sulfur compounds. This patent does not mention removal of BTX and necessarily produces DRI at low temperature, because coke oven gas, after passing through the iron ore bed is extracted from the reduction reactor and fed to its lower part where it is used for cooling the DRI before its discharge from said reactor. The coke oven gas exiting from the lower part of the reactor is cooled and fed to a reformer to reform the hydrocarbons, mainly CH4, to H2 and CO by reaction of the CH4 with CO2 in the presence of a catalyst. The principal object of this process is the desulfurization of coke oven gas before it passes through the catalytic reformer to avoid the catalyst poisoning by sulfur.
U.S. patent application No. 20090211401 describes a direct reduction process utilizing coke oven gas wherein the coke oven gas is mainly fed to the lower part of the reduction reactor wherein it is contacted with high temperature DRI produced in the upper part of the reactor. The DRI is cooled by the coke oven gas stream, and at the same time the DRI adsorbs the BTX and the sulfur compounds present in the coke oven gas; so that the gas withdrawn from the lower part of the reactor is clean of the undesirable contaminants and is driven to the reduction zone located at the upper part of the reactor wherein the reducing agents H2 and CO react with the iron oxides to produce the DRI. This process however cannot be used when high-temperature DRI is to be produced without any cooling; so as to take advantage of the thermal energy of the DRI when charged directly into the electric arc furnace thus achieving important economic advantages through the electric energy savings and by the increase of steel output productivity due to the shortening of the melting time and of the over all steel furnace heat time, in general.
The processes described in the above-cited patents all are limited with respect to the amount of coke oven gas that can be treated with the DRI produced in the reactor, because sulfur trapped in the DRI must afterwards be eliminated or its concentration minimized in the electric arc furnace where said DRI is utilized for steelmaking.
In contrast with the above limitation, applying the present invention a relatively small amount of DRI (or equivalent material) is used because it can become saturated with sulfur and properly disposed of without contamination of the DRI produced in the reactor.